1. Field of the Invention
The present invention relates to a dynamo-electric machine in which current regulation is performed by placing brushes in contact with commutator segments.
2. Description of the Related Art
FIG. 12 is a partial cross-section of a multi-polar, lap-wound, directcurrent electric motor being a conventional dynamo-electric machine. In this electric motor, a commutator assembly 101 is disposed in the vicinity of an armature 100.
The armature 100 comprises: a core 102 having slots extending longitudinally; and an armature coil 103 composed of conductor wound by a lap-winding method through the slots.
The commutator assembly 101 comprises: a plurality of commutator segments 104 arranged circumferentially; risers 106 disposed on end portions of the commutator segments 104; brushes (not shown) contacting the commutator segments 104; and equalizers 107 electrically connecting commutator segments 104 which are to have the same electric potential by means of the risers 106.
In the above electric motor, by supplying electric current to the armature coil 103 from outside by means of the brushes contacting the commutator segments 104, the armature 100 and commutator assembly 101 which are secured to a rotor shaft (not shown) rotate together with the rotor shaft due to electromagnetic action.
The equalizers 107 are disposed with the objective of preventing unbalanced currents from flowing through the brushes, and since they are connected to commutator segments which are to have the same electric potential, current does not normally flow through them. However, unbalanced phenomena may occur for a variety of reasons such as machining errors, and as a result, brush commutation sparks, etc., may be generated, and then current flows through the equalizers 107 to suppress these imbalances.
End portions 108 of the equalizers 107 shown in FIG. 13 are secured to the risers 106 by brazing, etc. The equalizers 107 are secured and supported by securing members 109, as shown in FIG. 12, enabling the equalizers 107 to withstand centrifugal force. Furthermore, like the armature coil, the equalizers 107 are normally composed of copper wire or a conductor, and the cross-sectional area of the conductor portion of the equalizers 107 is known to be best made about one half to one third of the cross-sectional area of the conductor portion of the wire of the armature coil 103 (See "Electrical Machines Compendium, Direct-Current Machines, Chapter 10 Armature Windings, 10.3 Lap Windings and Wave Windings, Institute of Electrical Engineers", or "Equalizing Coils and Commutation, RM-98-11, Institute of Electrical Engineers, Dynamo Workshop Data", for example).
In order to minimize the amount of material in the equalizers 107 or to reduce the space they occupy as much as possible, it is usual to connect points to be connected over the shortest possible distance while ensuring enough separation so that the equalizers 107 do not interfere with each other. Here, "points to be connected" refers to points on an armature circuit (constituted by the armature coil, the risers, and the commutator segments) which are always equipotential independent of their position on the armature 100, such as equalizer segments 104 separated by the same pitch as the pole pairs, for example.
The resistance values of the equalizers 107 in such cases will be explained using FIG. 14.
FIG. 14 is a diagram showing the risers and one of the equalizers 107 in the vicinity of the commutator 105 from the armature 100 side. As shown in FIG. 14, the average value of the radius of rotation of the connecting portions between the equalizer 107 and the armature circuits (in this case, the risers 106) is defined as r1 (m), and the pitch of the pole pairs is defined as .tau. (rad). Furthermore, when the resistivity of the armature coil 103 is .rho..sub.0 (.OMEGA..multidot.m) and the cross-sectional area of the conductor portions of the armature coil 103 is S (m.sup.2), if the value of the reference resistance R.sub.0 (.OMEGA.) is defined as EQU R.sub.0 =(.rho..sub.0 .multidot.(r1*.tau.)/(S/3)),
the value of the resistance of the equalizers R.sub.eq (.OMEGA.) which are composed of the same material as the armature coil is generally designed to be approximately twice that value at most. In other words, the value satisfies the expression EQU R.sub.eq .ltoreq.R.sub.0 *2. PA1 RB1&lt;&lt;RB3.
The electrical circuit in this case is shown in FIG. 15. FIG. 15 is an example in which equalizers 107 are disposed on all of the commutator segments 104 when there are four poles and twenty-two commutator segments. In a lap winding, it is usual to provide the same number of brushes as the number of poles, that is, four brushes 110 in the case of FIG. 15, two in each (positive and negative) polarity. In this case, if there are absolutely no causes for imbalance due to unbalanced machining, etc., the electric potential at both ends of all of the equalizers 107 is equal and no current flows through the equalizers 107. Furthermore, if the currents flowing through the four parallel circuits present under the main pole are given by i1, i2, i3, and i4, respectively, because the values of i1 and i2 are equal to those of i3 and i4, respectively, the values of the currents passing through each of the brushes 110 are all equal.
In a conventional direct-current electric motor such as that above, there may be differences between individual brushes 110 of the same polarity, and if an imbalance occurs between the voltage drops in the brushes 110 (voltage decreases resulting from resistance and contact resistance of the brushes 110), current flows through the equalizers 107, acting in a direction which makes the electric potential at both ends of the equalizers 107 equal.
FIG. 16 is a circuit diagram showing a case where there are differences between the contact resistances of individual brushes 110 and an imbalance has occurred between the parallel circuits. In the diagram, the contact resistances between the positive-side brushes 110 and the commutator segments are given as RB1, RB2, RB3, RB4, respectively, and the resistances of the equalizers are given as R.sub.eq 1 and R.sub.eq 2. Normally, RB1 and RB3, and RB2 and RB4, are constantly the same, and no currents flow though the equalizers R.sub.eq 1, R.sub.eq 2.
Now, let us suppose that there is an imbalance between RB1 and RB3, such that
If R.sub.eq 1 is sufficiently small, instead of the current i3 being supplied via the contact portion RB3 of the lower brush, it will be supplied via the contact portion (resistance value RB3) of the upper brush 110. As a result, the current passing through the upper brush 110 will be approximately three times the current passing through the lower brush 110, and the tolerable current value of the upper brush will be exceeded, not only giving rise to fusion of members peripheral to the brushes 110 due to the generation of heat by the upper brush 110, but in the end leading to fusion of the upper brush 110 itself. In order to prevent this, it has been necessary to take measures such as averaging out the differences between voltage drops in individual brushes 110 and reducing imbalances in the circuits by designing the brushes 110 with a greater current tolerance than necessary in order to be prepared for current imbalances resulting from differences between the contact resistances of individual brushes 110, or by disposing a plurality of brush segments 110a to a 110d in contact with the same commutator segment 104, as shown in FIG. 17.
Thus, in a conventional direct-current electric motor, one problem has been the necessity to ensure sufficient current tolerance in the brushes 110, or to increase machining precision in order to reduce individual differences, consequently leading to cost increases, size increases, and poor productivity.